July, 2012

Many would wonder why anyone would want to modify such a well-proven package like the Stealth Bomber or the Stealth Fighter and I suppose the simple answer is “because we can”. In my case I wanted to differentiate both my Bomber and Fighter from the standard offering, but in two completely different ways. In the case of the Bomber, the project was all about in your face grunt and speed…however with the Fighter, modifications were practically the inverse and they were all about making the E-bike as light and nimble as possible. Both E-bikes relied on the installation of Lithium Polymer batteries (LiPo) and custom controllers to achieve the desired results. Read on to see how these two very different objectives were reached.

Stealth Bomber modification and customization

My Stealth Bomber is the Mk-1 model and has a slightly bulkier frame then the current model. The Mk-1 Bomber was designed for Headway 10-Ah cylindrical cells, and as such it’s both wider and deeper in the battery compartment. In addition to this, it was built with a heavier gauge Cro-moly skin making this E-bike a few lbs. heaver than the current model. My Bomber also came with the higher torque (but lower speed) 5405 motor. This motor is good for around 37-MPH (60-kph) on the standard 80V setup.

My aim with this E-bike was to reduce its weight to around the same as the current model, but at the same time significantly increase the E-bike’s speed and acceleration. To achieve this, it meant the voltage needed a substantial increase, and as such both the battery pack and the controller would need to be replaced. My target speed was around 50-MPH but to achieve this with the high torque 5405 motor meant I needed to run around 120V.

Batteries

The first modification was to remove the stock Headway battery pack and replace it with some energy dense LiPo packs.  After some careful measurement, I found I could comfortably fit in fifteen of the 6S 5000-mAh Turnigy packs, and still have enough room for various plugs and fittings. The fifteen pack configuration was perfect for what I had planned, and meant I could arrange the packs into three groups of five. Each group were series-connected to create three separate 125V packs.

These grouped packs were then paralleled to create a 30S/3P configuration for a theoretical 1500-Whrs of total capacity. Packs were purchased from HobbyKing for a total cost of $850 landed, a cost basically covered by the sale of the second-hand Headway pack. It should be noted that the latest batteries used in the Stealth product are only slightly heavier then LiPo’s of the same capacity but with the added bonus of being fully protected by a BMS and completely safe to use.

Turnigy 6S / 5-Ah LiPo Battery packs arranged into 3 x 125V modules. A parallel wiring harness is then used to tie all 3 modules into a 30S/3P 1500 W/hr pack. Safe usable capacity is 1250W/hrs

Packs installed within the E-bike’s battery compartment.

Controller

The stock controller, although a robust high powered unit, could not handle the higher voltage without some major modification.  Rather than going through the process of changing out all the voltage sensitive parts such as FETs and capacitors, I opted to purchase a new Lyen 18-FET highway-edition speed controller. These controllers use an Infineon-based board but are then hand-assembled with high quality high-voltage componentry together with all the required connectors such as direct plug in Cycle Analyst, re-gen, ebrake, 3 speed switch and programming port.

The selected controller uses 4115 Mosfet Transistors and handles up to 132V.  However, the controller is setup for a 40A maximum output and as such still needs some modification to get some serious power out of them.  In this case my target power output was 80A. For the controller to stay reliable under these conditions modifications were definitely required. These included beefed up main current carrying traces, modifications to the current measuring shunt and upgrade of both battery and phase cables from 12-gauge to the fatter 10-gauge silicone cable. Custom programming was then required to setup the controller to suit.

Above you see the back side of the controller showing beefed up traces soldered by hand.

Software interface for programing custom parameters into the controller

Next step was to move the controller from the stock position under the front of the E-bike and mount it internally within the battery compartment. The controller was removed from its case and the controller heat-sink attached directly to the E-bike frame. By doing this, the frame now becomes a gigantic heat sink for the controller, allowing even better power handling performance. The removal of the controller from the outside also gives the E-bike a cleaner look, while at the same time fully protecting the controller from the elements.

Screws used to hold the controller heat sink in place. A layer of heat transfer paste was applied between the heat sink and E-bike frame to ensure the best possible heat dissipation.

Controller installed complete with protective cover over the delicate components.

Smoother overall lines without the externally mounted controller

External Battery Isolation

When running LiPo packs, I think it is of paramount importance to ensure the battery pack can be fully isolated from the controller without removing the side covers. To achieve this, an external loop cable system was installed from the battery to the controller.  This loop cable is external to the E-bike and unplugs in just a second, should the need arise. It also becomes both a bulk charging point and an auxiliary pack connection point. My connector of choice for this duty is the EC5. These plugs are typically used in high powered electric model aircraft however they make an excellent plug choice for high powered electric bikes also.

Top plug goes directly to the controller. An external auxiliary pack can be connected here if required. The lower plug goes directly to the battery and doubles as the charging point for the E-bike.

Loop cable attached. Doubles as an emergency quick disconnect battery isolation plug.

Bulk Charging

Bulk charging is a method of charging that’s used to charge the entire pack as one high-voltage unit, as opposed to splitting the pack down into smaller groups for balance charging. In this case I used a 900W LiFePO4 from BMS-Battery, that is set up to cut out at 125V.  This is a really quick and convenient way to charge LiPo’s however you need to be aware of the risks involved and take the necessary precautions to protect both yourself and property. These precautions are a subject all on their own, and as such I won’t go into detail in this review however it is extremely important that anyone attempting to use LiPo chemistry on an E-bike fully understand what they are getting in to.

900W charger from BMS Battery. This charges the battery pack in around 2 hours

3-speed switch

Having a 3-speed switch is nothing new to the E-biking community, however I think it is one of the most under rated accessories out there. On high-powered setups such as this E-bike, they make a huge difference to the E-bike’s ride-ability. Having 10-kW on tap means that the throttle becomes very touchy, and for someone who hasn’t ridden the E-bike before…its downright dangerous. I have speed-1 setup for around 22-MPH (35-kph), speed-2 around 34-MPH (55-kph) and speed-3 is unlimited and around 53-MPH (85-kph). On speed-1, the E-bike is very well mannered and basically idiot proof. Speed-2 accelerates at a similar rate to a stock Bomber but tops out earlier. Speed-3 is an absolute blast. With 10-kW on tap, the front wheel comes up very easily and acceleration is nothing short of brutal, even at upper end of the speed envelope. Acceleration is around zero to 45-MPH (72-kph) in 5 seconds.  This is quicker than most cars and even gives small motorbikes a bit of a scare.

Easily accessible 3-speed switch

Video depicting the awesome acceleration of the Stealth Bomber.

Other less obvious modifications are the replacement of the stock 16-tooth freewheel to a 14-tooth freewheel and the addition of a type of seat post where the rider can adjust the height while riding.

The 14-tooth freewheel allows you to still pedal at a comfortable cadence, even when traveling at 38-MPH (60-kph). Some would argue…what is the point of pedalling at these high speeds? The truth is, as a percentage, you are going to be contributing very little to the overall power required. However, being able to pedal gives you the ability to still get some exercise even when riding at high speeds. Personally it just feels wrong not to be pedaling with some meaningful input, no matter what the speed.

The adjustable seat post gives you the ability to adjust you seat height on the fly. This is particularly useful on E-bikes with big suspension travel. The Bomber has a massive 250mm travel at the rear and a total 14” clearance from the ground to the bottom bracket.  This means that if you want to have the correct leg extension while pedalling, you can only just touch the ground with your toes. This is fine for commuting, but when out in the dirt, you want to be low enough to easily drag a foot if you need too.  Having the adjustable seat post gives you the ability to quickly change to suit the conditions. As a tall guy, with long legs, I have found this addition to be particularly useful.

Stealth Bomber Summary

Although the Stealth Bomber is a great product out of the box, it also makes an excellent candidate for higher performance modifications. This is due to its extremely robust construction, high speed handling capabilities, and strong braking performance.  The Mk-1 Bomber, dare I say it, is an even better candidate for modification than the latest model. I say this because firstly, it is out of warranty so you don’t feel guilty about making changes to it. Secondly, it has a larger battery compartment and uses thicker materials for the frame than the current model. Not to say the current model is under-engineered, it’s more that the Mk-1 model was perhaps a little over-engineered. Perfectly understandable for a new product offering a lifetime warranty on the frame and swing arm.

With the modifications now complete, total weight has been reduced to just under the weight of the standard Bomber, tipping the scales at only 51 kg (112-lb) Power has been effectively doubled over the stock setup, and the overall look is improved with the relocation of the controller. Range is similar to the stock Bomber if ridden conservatively, but spirited riding will certainly reduce you overall range quickly.

Kepler-modified Bomber Completed

Stealth Fighter modification and customization

The Stealth Fighter project was a completely different proposition to the Stealth Bomber Project. Firstly, my Bomber was essentially out of its warranty period and as such I had a free rein to basically do what ever I wanted. The Fighter was quite a different proposition. This project started with a brand new frame from the Stealth Factory. Stealth Bikes do not sell frames to the general public, however…my long time association with the company gave me enough leverage to convince management to sell me a frame, but with a few specific conditions attached.

The primary aim of this project was to build a Stealth Fighter with the very best balance of weight, power and range possible with essentially off the shelf parts. The Fighter frame is in my opinion the perfect base platform for this type of project. It has been designed from the ground up to be massively strong and rigid, but at the same time relatively light. If you put stock mountain bike wheels on this E-bike and deleted the battery, I suspect it would make a seriously capable downhill bike. Unlike converting a down hill bike to an E-bike, this frame is specifically designed to be electric. This means the batteries are fully enclosed in a unique Cro-moly monocoque frame, and the swing arm is engineered to handle a high-powered hub motor without fear of the motor ripping out of the dropouts under extreme load.

Goals for this build were as follows:

Main component selection was as follows:

Controller

First item on the shopping list was the controller. I had bee,n testing a modified 6-FET controller on one of my other E-bikes for some time now with some very good results. The results were good enough for me to be confident in using the same controller on this build. The unit uses an Infineon main board populated with 100V capable componentry.  This includes super-efficient 4110 FETS and 100V capacitors throughout. Battery and phase cabling are 12-gauge wire with silicone insulation and all the required plugs such as a direct Cycle Analyst connection, 3 speed switch, throttle, and programming point are fitted to the tiny board. Drop resistor values for low voltage supply to the controller are optimized for battery voltages of between 60V and 86V.

As with the modified Bomber, the controller was mounted directly to the  frame via the controller’s heat sink. The Fighter layout actually suited the internal controller installation quite well with a nice unused section of frame available just above the battery pack. A protective aluminium panel was then installed just below the controller together with an aluminium central divider plate for the battery packs.

Motor

Next on the shopping list was the geared hub motor with a Bafang BPM code-10 motor selected for the job. The code-10 winding is rated at 8.2-RPM per volt. A quick calculation confirmed that 40-MPH (64-kph) was achievable on a 70V / 18S nominal setup.  The motor was laced into a stout 24” downhill rim using good quality 14-gauge stainless steel spokes and nickel-plated brass nipples.

Mounting the new wheel assembly into the Fighter swing arm proved to not be as straightforward as hoped.  The Fighter has 150mm wide rear dropouts, with the Bafang being the standard 135mm.  This meant sending the wheel and swing arm back to the Stealth factory for reworking. Two weeks later, the modifications were complete including re-powder coating of the swing arm assembly.

Stealth factory modified swing arm

Batteries

Batteries were next on the list with 3 x Zippy 6S / 8-Ah LiPo battery packs ordered for the job once again from HobbyKing. This pack has a theoretical capacity of 600W/hrs, however the real world operation is closer to around 450-Whrs. For $300 landed, still not too bad.  The 3 packs were series connected to create an 18S/1P configuration. The pack was then installed in the E-bike’s lower battery compartment so as to optimize the front to back balance. As pictured below, the upper battery compartment is used for a second 18S pack to extend the range when required.

As with the Bomber, the battery and controller can be isolated from outside of the E-bike without removing a side cover. In this case, the battery to controller isolation is a little less elegant with the two leads simply exiting the frame just in front of the seat post.  The loop cable then tucks nicely within the seat post support framework, once it’s plugged together.

Bulk Charging.

Since the Fighter uses a much smaller battery then the Bomber, a much smaller bulk charger could be selected. In this case I used a 400W LiFePO4 from BMS-Battery, set up to cut out at 75V.  This charger is compact enough to store on the E-bike in a handlebar bag and still leave plenty of room for my wallet, keys, and phone. Again I want to stress that that although this is a really quick and convenient way to charge LiPo’s, you need to be fully aware of the risks involved and take the necessary precautions to protect both yourself and property.

Display

A large screen Cycle Analyst is used and mounted on the head stem instead of the smaller screen unit’s stock position that’s built into the frame. Personally I found the CA a little difficult to read on the stock Fighter mainly due to rake and angle of the frame. Having the large screen up on the head stem makes it very easy to read the display and see exactly what’s going on without taking too much attention away from what’s going on in front of you.

Human input

For human input, the E-bike uses the standard Schlumpf 2-speed bottom bracket planetary overdrive system. A 7-speed cluster was also fitted in readiness for possible future installation of a derailleur, however for now, just the 11-tooth gear is used. Having the two speeds is certainly adequate for commuting duties and the two available ratios work quite well with a comfortable cadence of 25-kph to 30-kph in direct drive, and a comfortable cadence of 45-kph to 50-kph in overdrive.

Braking

Braking duties are taken care of by a set of Magura MT-2 Hydro’s clamping down on 203mm discs back and front. These brakes are the best I have used to date, and they have an amazing feel and stopping power. One or 2 fingers are all that are need to pull the bike up. I highly recommend these brakes not only for their excellent performance, but also in terms of value for money.

Throttle and 3-Speed switch

As with all my E-bikes, I use a 3-speed switch to manage the behaviour of the bike. However, in this case I use the 3-speed switch in conjunction with a thumb throttle, as apposed to a half twist throttle usually found on Stealth Bikes. With the power level of this E-bike, I tend to set the speed limit with the 3-speed switch, then just mash the thumb throttle to the end stop.  I find this very comfortable and less fatiguing as you are not constantly adjusting the throttle to maintain a certain speed. Of course I can still modulate the speed quite easily when required.

Speed-1 accelerates smoothly and tops out at 20-MPH (32-kph).  This fits in really well with first speed of the Schlumpf and gives the E-bike excellent range in this mode. Speed-2 sees around 30-MPH (48-kph) and ties in well with the overdrive ratio of the Schlumpf. I tend to use this speed the most as it’s a good compromise between range and speed.  Speed-3 is of course unlimited and allows the E-bike to wind out to its maximum speed of around 38-MPH (60-kph).

Riding experience

The controller has been setup to provide around 2000W output. With a geared hub motor, 2000W feels very lively, especially off the mark. I think the bike would be quicker off the mark than a standard 3000W Fighter, however the stock Fighter would most likely reel it back in through the midrange. The E-bike feels incredibly light on the road and changes direction very quickly. I find I can brake really late into corners then, punch out of the corner at full throttle with full confidence.

Economy wise, the geared hub motor is certainly an improvement over the stock direct drive motor. I am seeing figures between 20% and 40% over the stock motor. However if pushed really hard, economy drops off to a similar level. This E-bike has been setup mainly for urban duties as opposed to the stock Fighter that has been designed more for off-road duties. The Bafang motor at 2000W is coping extremely well, with zero issues to date.  Although very little off-road work has been done with the bike, tests to date have found the bike to be very capable in the rough stuff also.

It was surprising though, how quickly the efficiency dropped off when pushed hard. Efficiency wise, I think the stock Fighter motor would overtake the Bafang motor under these conditions. I think it would also be fair to surmise that the stock Fighter motor would be more durable when used under these conditions. Fighter acceleration test:

Fighter Vs Bomber

So which E-bike do I prefer? The Monster Bomber or the Super Sharp Fighter? Although both have their merits, it’s the Fighter that wins out for me. I just love the fact that it still feels like a bike, and you can actually pedal it without power at reasonable speeds and reasonable distances. Although it doesn’t accelerate like a train, it still gives quite a reasonable kick, especially off the mark. Its speed is also more then adequate without going into an area reserved for motorbikes and motorbike safety equipment. The Bomber however, is far from retired and is now used mostly for dirt track duties. The bike is basically unstoppable and climbs like a Billy Goat. Also the big battery capacity gives you plenty of range even when pushing hard.

My biggest problem now is choosing which E-bike to ride…not a bad problem to have though!

Story Submitted by John “Kepler” Wessel of Melbourne Australia

Editor Note:  Kepler  is  a well respected guru in the electric bike community who on his spare time  invented his own friction drive system “the E-boost,”  a clean and efficient system. Also read Kepler’s review of the Stealth Fighter here.